Porous materials namely mesoporous and macroporous materials are interesting classes of materials useful for various practical applications. Kresge, C. T.; Leonowicz, M.; Roth, W. J.; Vartuli, J. C.; Beck, J. C., Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature, 359, 710 (1992); Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonardo, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; Van Driel, H. M., Large-scale synthesis of a silicon photonic crystal with a complete 3-dimensional bandgap near 1.5 micrometers, Nature, 405, 437 (2000). The discovery of periodic mesoporous silica materials denoted M41S or MCM-type having pore sizes 2-10 nm in 1992 represented a paradigm shift in the synthesis of porous inorganic materials with a structure founded upon a regular array of monodispersed pores in the mesoscale size range of 2-50 nm defined by the International Union of Pure and Applied Chemistry IUPAC convention for porous solids.
To amplify, the materials were prepared in a straightforward synthesis that involved the aqueous phase co-assembly and acid or base catalyzed hydrolytic poly-condensation of silicate-surfactant micelles followed by removal of the surfactant by thermal or chemical or photochemical post-treatment steps. This synthesis strategy created a silica replica of the templating micelles (a silicatropic mesophase) and represented a new way of creating silica materials with crystalline mesoporosity having a pore architecture (e.g., hexagonal, cubic, worm-hole) and pore dimensions (range of diameters 2-10 nm) that were predicated upon the structure and dimensions of the surfactant-directing micelle assembly. Using this synthetic approach the mesopore dimensions could be chemically controlled with angstrom precision anywhere in the range of 2-10 nm. In a creative extension of this strategy, researchers at the University of California at Santa Barbara demonstrated that by using tri-block copolymer micelles, involving for example the co-assembly of a polypropylene oxide-b-polyethylene oxide-b-polypropylene oxide mesophase with silicate precursors, as a new and larger dimension templating mesophase, then the mesopore size range of the MCM41 class of periodic mesoporous silica materials could be boosted to the upper mesoscale range of 10-30 nm to create a new class of much larger mesopore silica materials that were denoted SBA periodic mesoporous silicas, see—Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky G. D., Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores, Science 279, 548 (1998).
It is important to note that the channel walls of all of these MCM and SBA classes and structure types of periodic mesoporous silicas were glassy having just short range order, the channel walls lacked structurally well-defined silica sites like those found in zeolites (a class of solids defined as crystalline microporous aluminosilicates), and were found to be devoid of useful channel functionality for perceived applications that could benefit from the size and shape controlled mesopores and specific adsorption properties of the materials. In other words while the mesopores in MCM41, MCM48 and SBA materials were monodispersed (single size) and the mesoporosity could be either periodic (hexagonal, cubic) or randomly organized (worm hole—see Linssen, T.; Cassiers, K.; Cool, P.; Vansant, E. F., Mesoporous templated silicates: an overview of their synthesis, catalytic activation and evaluation of the stability, Adv. Coll. Interf. Sci., 103, 121 (2003)), the material behaved more or less like any other form of porous silica derived from say silica sol-gel type chemistry, exemplified by well-known classes of materials called xerogels and aerogels, and that contained a random spatial distribution of different diameter mesopores in a glassy silica matrix. Hence the envisioned benefits of this new class of periodic mesoporous silica MCM41, MCM48 and SBA materials were never really realized in practice and to the best of our knowledge no products or processes have emerged in the more than ten years since their discovery.
Since their discovery, tremendous efforts have been devoted to overcome the functionality deficiency of the MCM41, MCM48 and SBA class of mesoporous silica materials by, for example, incorporating other elements into the materials, creating entirely different compositions, crystallizing the constituents of the channel walls, and of particular relevance to the present invention, by incorporating useful organic functionality into the materials, see—Asefa, T.; Ozin, G. A.; Grondey, H.; Kruk, M.; Jaroniec, M., Recent developments in the synthesis and chemistry of periodic mesoporous organosilicas, Studies Surf. Sci. Catal. 141, 1 (2002). In the context of the latter direction of investigation, two main methods of building organic function into periodic mesoporous silica to create organic-functionalized mesoporous materials have been devised. The first involving a three-step process based upon first synthesizing by template directed means a periodic mesoporous silica, second removing said template from the as-synthesized periodic mesoporous silica and third grafting organo-functionalized alkoxysilanes RSi(OEt)3 to channel surface silanol groups SiOH to give the desired organic-functionalized mesoporous materials. In the second approach, instead of a three-step process a one-step method is utilized to obtain the desired periodic mesoporous organosilica that involves co-assembly of organo-functionalized alkoxysilanes RSi(OEt)3 with alkoxysilanes Si(OEt)4, see—Stein, A.; Melde B. J.; Schroden, R. C. Hybrid inorganic-organic mesoporous silicates—Nanoscopic reactors coming of age Adv. Mater. 12, 1403 (2000). The organic groups utilized in the precursors RSi(OEt)3 used for both of these synthetic approaches, which ends up in the desired periodic mesoporous organosilica product is “terminally” bound to the silicon atom in said precursors and said product. Whichever synthetic strategy is used to make these organo-functionalized mesoporous materials with organic groups terminally bound to the walls of the channels, the surfactant template can be removed from the material by thermal or chemical or photochemical post-treatment steps. In this context of template removal from said as-synthesized organic-functionalized mesoporous materials, thermal usually means heating in air or oxygen to oxidatively remove said template from said as-synthesized organic-functionalized mesoporous materials under conditions that do not destroy said terminal organic function; photochemical usually means irradiating said template containing organic-functionalized mesoporous materials with ultraviolet light in air or oxygen to photooxidatively remove said template from said as-synthesized periodic mesoporous silica under irradiation conditions that do not destroy said terminal organic function; chemical usually means reacting said template containing organic-functionalized mesoporous materials with a reagent that serves to chemically remove said template from said as-synthesized periodic mesoporous silica under conditions that do not destroy said terminal organic function.
Another way of incorporating this kind of terminally bound organic functionality to the pore walls of the periodic mesoporous silica is a modification of the three-step grafting procedure described above but is instead reduced to a two-step process by circumventing the template removal step. This is achieved by adding the organo-functionalized alkoxysilane RSi(OEt)3 directly to the as-synthesized template-containing periodic mesoporous organosilica whereby it is able to simultaneously displace the imbibed template from the periodic mesoporous silica and at the same time anchor to silanol groups on the pore wall of the periodic mesoporous silica as a terminally bound organic group through a single chemical linkages, see—Valentyn, A.; Mietek, J., Simultaneous modification of mesopores and extraction of template molecules from MCM-41 with trialkylchlorosilanes, Chem. Commun, 23, 2373 (1999) and Liu, Y.-H.; Lin, H.-P.; Mou, C.-Y., Direct method for surface silyl functionalization of mesoporous silica, Langmuir, 20, 3231 (2004).
To emphasize, in all three cases mentioned above the desired organic function in the resulting periodic mesoporous organosilica material is chemically bound to the surface of the channel walls as a “terminal organic group through a single chemical linkage” and protrudes into the channel and hence to some extent occupies channel void space and also serves to block the passage of guests in the channels. Moreover the distribution of organic functional groups within the channels is not strictly under control and effects of localized anchoring in the pore mouths of the mesopores and phase separation of precursors, results in inhomogeneous patterns of organic groups on the surface of the channel walls.
A creative and inventive way to circumvent all of these problems with surface attached terminal organic groups on the channel walls of periodic mesoporous organosilicas was reported simultaneously and independently by three research groups in 1999: at the University of Toronto, see—Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Periodic mesoporous organosilicas with organic groups inside the channel walls, Nature, 402, 867 (1999); the University of Minnesota, see Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A., Mesoporous sieves with unified hybrid Inorganic/organic frameworks, Chem. Mater. 11, 3302 (1999); and Toyota Research Laboratories, see—Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T, Terasaki, O. Novel mesoporous materials with a uniform distribution of organic groups and inorganic oxide in their frameworks, J. Am. Chem. Soc., 121, 9611 (1999). Toyota Corporation obtained a patent on this class of periodic mesoporous organosilica materials—Inagaki, S.; Guan, S.; Fukushima, Y. Organic/inorganic materials including complexes of porous materials. Ger. Offen. (2000), DE 19930944.
It involved the use of a new type of silica precursor in the surfactant templated co-assembly process, called silsesquioxanes exemplified by (EtO)3SiRSi(OEt)3, in which the organic function R instead of being present as a terminally bonded group to the alkoxysilane as referred to above is rather positioned as a “bridging group” between two alkoxysilyl groups. The resulting templated material is called a periodic mesoporous organosilica (PMO) in which the bridging organic group R is exclusively integrated into the silica framework to create organosilica channel walls. This represents a distinct situation to the earlier generation of organic-functionalized mesoporous materials mentioned above in which the organic groups are exclusively terminally bound to the surface of silica channel walls and protrude into the channel spaces. In the PMOs by contrast, the mesopores exist in an organosilica matrix built entirely from bridging O1.5SiRSiO1.5 moieties. Furthermore, by performing the same kind of template directed synthesis but instead using a controlled ratio of the silsequioxane and alkoxysilane precursors in the co-assembly process, it proved possible to access an entire family of PMO materials in which the loading of the bridging organic group in the organosilica channel walls can be varied between 100% and 0%. In this way it proved feasible to fine-tune the chemical and physical, mechanical and electrical, optical and electronic properties of the PMOs anywhere between the two end members with high fidelity. Other extensions involved the synthesis of novel classes of PMOs containing two or more distinct types of bridging organic groups or indeed PMOs with both bridging and terminal organic groups, see—Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A., Novel bifunctional periodic mesoporous organosilicas, BPMOS: synthesis, characterization, properties and in-situ selective hydroboration-alcoholysis reactions of functional groups, J. Am. Chem. Soc., 123, 8520 (2001).
Still other extensions involved the use of [(EtO)3Si]nR silsesquioxane precursors in which n=3, 4 to yield PMOs in which the bridging organic group spanning the alkoxysilane groups can be triply or quadruply bonded and the organic content can be cross-linked to increase its thermal stability, see—Kuroki, M.; Asefa, T.; Whitnal, W.; Kruk, M.; Yoshina-Ishii, C.; Jaroniec, M.; Ozin, G. A. Synthesis and properties of 1,3,5-benzene periodic mesoporous organosilica (PMO): novel aromatic PMO with three point attachments and unique thermal transformations, J. Am. Chem. Soc. 124, 13886 (2002).
All of these breakthroughs essentially relegated the MCM and SBA classes of periodic mesoporous silica materials, which were devoid of useful functionality, to be the pure silica end-members of an enormous family of PMOs brimming with valuable functionality that could be orchestrated synthetically through the choice of the bridging organic group R to target a particular application.
The ability to directly include, in a predetermined fashion, bridging organic groups into the silica walls of a periodic mesoporous silica created an entirely new class of PMO nanocomposites, synthesized from the “bottom-up” and with “molecular scale” control, and which offered a myriad of envisioned opportunities based upon the ability to utilize organic synthetic chemistry to control the chemical and physical properties of the material.
The ability to directly include, in a predetermined fashion, bridging organic groups into the silica walls of a periodic mesoporous silica created an entirely new class of PMO nanocomposites, synthesized from the “bottom-up” and with “molecular scale” control, and which offered a myriad of envisioned opportunities based upon the ability to utilize organic synthetic chemistry to control the chemical and physical properties of the material.
The trend setting reports of the PMOs from the three inventor groups nevertheless inspired groups around the world to get involved in the materials and extend the research. Hundreds of papers have now appeared on PMOs (ISI lists more than 250 citations) and are beginning to demonstrate that diverse kinds of organic functionality can be incorporated into the materials, which can likely be exploited to advantage in a number of application areas including but not limited to catalysis and separations, chemical sensing, bioassays and controlled release of chemicals and drugs, microelectronic packaging and digital printing.
In related drug delivery applications, several papers are also now appearing in the literature where, for instance, the release of an inflammatory drug ibuprofen from mesoporous materials have been demonstrated; see—Vallet-Regi, M.; Rámila, A.; del Real, R. P.; Pérez-Pariente, J. A New Property of MCM-41: Drug Delivery System Chem. Mater. 3, 308 (2001).
There are, however, some serious deficiencies with the PMOs that are problematical with respect to a number of perceived areas of application s earmarked for the PMOs. These include the fact that a significant fraction of the bridging organic groups are buried within the internal regions of the organosilica channel walls and are therefore not able to be exploited in a number of applications that require them to be spatially accessible on the surface of the channel walls. These so-called inaccessible organics are essentially wasted in a chemical sense and serve only to make the cost of production of the PMO materials higher than it would have been had they not existed. Another problem concerns the geometric and steric constraints imposed on the bridging organic groups that are accessible on the channel wall surface of the PMO but not necessarily optimally aligned to exploit for example its adsorption and reactivity properties. Also, not all (EtO)3SiRSi(OEt)3 silsesquioxane precursors are able to successfully assemble into PMO materials because of either competing and unwanted intramolecular cyclization reactions, unfavorable hydrolytic poly-condensation kinetics or collapse of the resulting PMO because the bridging organic group is insufficiently rigid to support the desired mesostructure.
It would be very advantageous to provide a method of producing an entirely new class of hybrid porous organic-metaloxide (HPO) materials that have all the desired attributes of the PMOs but are able to overcome deficiencies of the type mentioned but not limited by the ones mentioned above. Thus, an objective of the present invention is to provide a new class of hybrid porous organometaloxide (HPO) materials that have at least some of the desired attributes of the PMOs but are able to overcome the aforementioned deficiencies of the PMO materials.